Magnetic resonance imaging (mri) using spir and/or chess suppression pulses

ABSTRACT

A magnetic resonance imaging (MRI) apparatus includes an MRI imaging condition setting unit configured to set an imaging condition frequency-selectively applying a first suppression pulse for suppressing fat and further frequency-selectively applying a second suppression pulse to the fat after applying the first suppression pulse, a slip angle of the second suppression pulse differing from that of the first suppression angle, and the second suppression pulse further suppressing remaining fat after applying the first suppression pulse. The image data acquisition unit acquires image data according to the imaging condition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of co-pending application Ser. No. 12/662,266 filedApr. 8, 2010, which is a division of Ser. No. 12/052,274 filed Mar. 20,2008 (now U.S. Pat. No. 8,076,935 issued Dec. 13, 2011), which claimspriority under 35 U.S.C. §119 based on Japanese Patent Application No.2007-081640 filed Mar. 27, 2007, the entire contents of all of which areincorporated herein by reference.

BACKGROUND

1. Technical Field

The present invention relates to an MRI (magnetic resonance imaging)apparatus and a magnetic resonance imaging method that radio excitesnuclear spins of an object magnetically with an RF (radio frequency)signal having the Larmor frequency and reconstructs an image based on anNMR (nuclear magnetic resonance) signal generated due to the excitationand, more particularly, to a magnetic resonance imaging apparatus and amagnetic resonance imaging method that make it possible to image under asuppression method for suppressing a target signal from a material suchas fat or silicone.

2. Description of Related Art

Magnetic resonance imaging is an imaging method that excites nuclearspins of an object set in a static magnetic field with an RF signalhaving the Larmor frequency magnetically and reconstructs an image basedon an NMR signal generated due to the excitation.

In the field of magnetic resonance imaging, there is a fat saturationmethod to acquire signals while suppressing signals from fat (fatsignals). Conventionally, fat saturation methods used widely in generalinclude CHESS (chemical shift selective) method, SPIR (spectralpresaturation with inversion recovery) method (called SPECIR as well)and STIR (short TI inversion recovery) method.

In fat saturation methods, the CHESS method is called a selective fatsaturation method since the CHESS method is a method to suppress onlyfat signals frequency-selectively using 3.5 ppm difference in resonantfrequencies between water protons and fat protons (see, for example,Japanese Patent Application (Laid-Open) No. 7-327960, Japanese PatentApplication (Laid-Open) No. 9-182729 and Japanese Patent Application(Laid-Open) No. 11-299753).

FIG. 1 is a diagram explaining a method for suppressing fat signalsunder a conventional CHESS method.

In FIG. 1, the abscissa denotes a frequency and the ordinate denotes asignal intensity of NMR signal. In a frequency selective fat saturationmethod, a static magnetic field is made uniform by shimming prior toimaging. As shown in FIG. 1, respective protons of water and fat have3.5 ppm difference in a resonant frequency and, therefore, a peak offrequencies of fat signals becomes sharp when the uniformity of a staticmagnetic field is obtained enough by shimming. Then, when a fatsaturation RF pulse with a 90 degree flip angle (FA) and a frequencymatching a resonant frequency of fat, i.e., a CHESS pulse is applied totilt only longitudinal magnetization of fat protons by 90 degreefrequency-selectively and, subsequently, magnetization of fat protonssubstantially disappears by using a spoiler pulse previous to dataacquisition, fat signals can be suppressed. As described above, theCHESS method is effective in fat saturation on a region with highuniformity with regard to a static magnetic field.

The SPIR method is also a selective fat saturation method that uses adifference in resonance frequencies between water protons and fatprotons (see, for example, Japanese Patent Application (Laid-Open) No.2006-149583).

FIG. 2 is a diagram explaining a method for suppressing fat signalsunder a conventional SPIR method. FIG. 3 is a diagram showing a TI forapplying a pulse for excitation after applying the SPIR pulse shown inFIG. 2.

In FIG. 2, the abscissa denotes a frequency and the ordinate denotes asignal intensity of MR signal. Further, in FIG. 3, the abscissa denotesan elapsed time after applying an SPIR pulse and the ordinate denotes alongitudinal magnetization z of a proton spin.

In the SPIR method, an SPIR pulse that is a frequency-selectiveinversion RF pulse having a frequency matching with a resonant frequencyof fat signals is applied. A FA of an SPIR pulse is set from 90 degreesto 180 degrees. When an SPIR pulse is applied, the proton spins in fatmagnetized by a static magnetic field tilt by the angle according to theFA and a longitudinal magnetization z of the proton spins in fat shows aminus value. Accordingly, the longitudinal magnetization z of protonspins increases with time to show a plus value by longitudinalrelaxation (T1 relaxation). Then, with setting an inversion time (TI) tothe timing which a longitudinal magnetization z in fat reaches the nullpoint by T1 relaxation after applying an SPIR pulse, an RF pulse forexcitation of water signals is applied. This allows only water signalsto be excited selectively.

In the SPIR method, if an angle to tilt proton spins in fat, i.e.,suppression effect is set largely like the case that a FA of an SPIRpulse is set to 180 degrees, for example, to suppress fat signals iseasy even in a fat region having a resonant frequency band with a broadbase. Thus, since a FA of an SPIR pulse is larger than a FA of 90 degreepulse, an SPIR pulse has a larger frequency band to be able to suppressfat signals than that of a normal CHESS pulse. In the SPIR method, sincethere is a dead time TI until the application timing of an RF pulse forexcitation after to an application of an SPIR pulse, the waveform of anSPIR pulse can be better approximated to a rectangle.

In the SPIR method, though it is necessary to acquire data after a TIwhich is the time until an intensity of fat signal (a longitudinalmagnetization z of a proton) becomes the null point, the TI can be alsoshortened by reducing a FA of an SPIR pulse. Generally, a TI isapproximately from 150 ms to 180 ms in a 1.5T MRI apparatus and a TI isapproximately from 200 ms to 250 ms in a 3T MRI apparatus.

On the other hand, the STIR method is a fat saturation method to use adifference in T1 relaxation times between fat and water signals and is anon-selective fat saturation method. Therefore, in the STIR method,shimming is unnecessary. In the STIR method, a region including a fatregion and a water region is excited by a STIR pulse with a 180 degreeFA. As shown in FIG. 2, the T1 relaxation time of water shown with thedotted line is longer than the T1 relaxation time of fat. Then, thetiming which a longitudinal magnetization z in fat protons becomes thenull point is set to the TI and an RF pulse for excitation is applied.Then, after a lapse of the TI, a longitudinal magnetization z of waterprotons does not reach the null point yet and only water protons can beexcited selectively.

However, since a CHESS pulse is an RF pulse with a 90 degree FA, whenuniformity of static magnetic fields in a water region and a fat regionis not satisfactory, fat signals cannot be suppressed sufficiently.

FIG. 4 is a diagram showing an example case of insufficiently suppressedfat signals in case of performing fat-saturation under a conventionalCHESS method.

In FIG. 4, the abscissa denotes a frequency and the ordinate denotes asignal intensity of NMR signal.

As shown in FIG. 4, when the uniformity of the static magnetic fielddoes not become satisfactory even if shimming is performed, the base ofa resonant frequency in fat becomes wide. Specifically, in a regionshowing a large susceptibility such as a breast and a jaw, there is atrend in which a band of fat signals extends in the frequency direction.Therefore, even if a CHESS pulse with a 90 degree FA is applied,component of fat signals remains as indicated by arrows. Thus, the CHESSmethod has a disadvantage that it is difficult to sufficiently suppressfat signals extending in a wide frequency band in the case of an unevenstatic magnetic field.

FIG. 5 is a tomographic image of an object obtained with a non-uniformstatic magnetic field under a conventional CHESS method.

As shown in FIG. 5, in the case of an uneven static magnetic field, itis recognized that fat signals are not suppressed sufficiently and a fatregion is depicted on a tomographic image of an object.

In the conventional SPIR method, when a longitudinal magnetization offat protons does not become the null point with the result that theadjustments of a TI and a FA of fat saturation pulse are notappropriate, unsuppressed fat signals remain. Note that whether or notfat signals show null changes depending on a T1 value of fat. Therefore,when it is assumed that a T1 of fat value and TI are fixed, adjustingfor a FA of a fat saturation pulse allows fat signals to show nulllogically.

However, practically, fat signals often remain by the factors such asnon-uniformity of a magnetic field, non-uniformity of an RF power (FA)of a fat saturation pulse and a subtle difference in a T1 value of fatwithin a living body.

FIG. 6 is a diagram showing an example case of fat signals remaining dueto an inadequately adjusted TI or FA of a fat-saturation pulse under aconventional SPIR method.

In FIG. 6, the abscissa denotes a frequency and the ordinate denotes asignal intensity of NMR signal. In the SPIR method, in the case that theadjustment of a TI and a power of a fat saturation pulse is incompleteor by the factors such as influence of non-uniformity of a magneticfield and an RF power (FA) of a fat saturation pulse and a subtledifference in a T1 value within a living body even if a TI and a powerof a fat saturation pulse are adjusted, fat signals remain as shown inFIG. 6. Further, the SPIR method has the disadvantage to obtainsufficient fat saturation effect at only a region showing a highuniformity with regard to a static magnetic field like the CHESS method.

FIG. 7 is a tomographic image of an object obtained with an inadequatelyadjusted FA of a fat-saturation pulse under a conventional SPIR method.

FIG. 7 shows an image obtained by setting a TI to be the shortest andadjusting a FA of a fat saturation pulse so that fat signals show null.As shown in FIG. 7, the fat saturation effect by the conventional SPIRmethod is better than that by the CHESS method. However, it isrecognized that a fat region is depicted on the tomographic image of anobject without sufficiently suppressing fat signals by the factorsdescribed above even if a FA of a fat saturation pulse is adjusted.

Although shimming is unnecessary since the STIR method is not a fatsaturation method to suppress fat signals frequency-selectively,intensities of water signals to be acquired become low due toacquisition of the water signals subsequently to a lapse of a TI.Therefore, in the STIR method, there are problems that the SNR(noise-to-signal ratio) decreases and a decrease of the SNR leads toextension of an imaging time.

As mentioned above, in a conventional frequency selective fat saturationmethod, when uniformity of a static magnetic field cannot be obtainedsatisfactorily due to the influence of the shape of an imaging part likethe case of imaging a region such as a jaw part and a breast, it isdifficult to suppress fat signals sufficiently. On the other hand, in aconventional non-frequency selective fat saturation method, theintensities of water signals to be acquired decrease and an imaging timebecomes long.

In addition, in the conventional frequency selective fat saturationmethod, when a parenchymal part of an object is to be suppressed, pluralexcitation pulses are applied repeatedly under the condition that eachfrequency of excitation pulse is matched with a resonant frequency ofwater. However, in the case of a high-speed imaging, an applicationinterval of an excitation pulse becomes long and signal recovery from animaging target occurs due to the influence of a longitudinalmagnetization, Consequently, the suppression effect in the frequencydirection by an excitation pulse does not become constant and thesuppression effect of signals cannot be obtained sufficiently dependingon a set frequency of an excitation pulse.

That is, though the suppression effect of fat signals can be evaluatedby a slice profile showing signal intensity to a frequency variation ofan excitation pulse, the profile changes according to an applicationinterval of an excitation pulse. For example, in the case that dataacquisition is performed using a sequence of FFE (fast field echo) typeunder the segment k-space method, if the number of segments isincreased, the profile showing the suppression effect of fat signals isimproved since an application interval of an excitation pulse becomesshort; on the contrary, if the number of segments is decreased, asatisfactory profile cannot be obtained since an application interval ofan excitation pulse becomes long.

Note that the segmented k-space method is a data acquisition method thatsegments data in k-space into several areas (segmentalization) andretrieves data from every segment sequentially. Therefore, the number ofsegments is equivalent to a number of divisions of phase encoding (PE)in k-space.

FIG. 8 is a diagram showing a profile representing a simulation resultof the effect by conventional fat-saturation methods.

In FIG. 8, the abscissa denotes a frequency shift amount (ppm) of anexcitation pulse from a resonance frequency of fat and the ordinatedenotes a relative signal intensity of an echo signal in case of settingthe maximum values as 1.

As shown in FIG. 8, it is confirmable that the profile in the case thatthe number of segments is two has more deteriorated stability of fatsaturation effect in the frequency direction than the profile in thecase that the number of segments is sixty-four since an applicationinterval of an excitation pulse in the case that the number of segmentsis two is longer than that in the case that the number of segments issixty-four.

Therefore, adjusting a waveform of an excitation pulse is needed inorder to improve a slice profile showing the fat saturation effect. Forexample, when an excitation pulse has a sinc waveform, a waveformcontrol such as expansion and contraction of an excitation pulsewavelength and change of a side lobe shape are necessary.

Though signals from an arbitrary matter such as water and silicone aswell as signals from fat can be also suppressed, there is a problem asdescribed above in the case that signals from a matter except fat aresuppressed as well.

BRIEF SUMMARY

The technology described below has been made in light of conventionalsituations, and it is an object of the exemplary embodiments to providea magnetic resonance imaging apparatus and a magnetic resonance imagingmethod that make it possible to perform frequency-selective-suppressionfor acquiring a signal that should be acquired with higher signalintensity while more satisfactorily suppressing a signal that should besuppressed.

The exemplary embodiments provide a magnetic resonance imaging apparatuscomprising: an imaging condition setting unit configured to set animaging condition applying a first suppression pulse and a secondsuppression pulse of which at least ones of types, center frequenciesand frequency bands are different from each other, the first and thesecond suppression pulses each frequency-selectively suppressing atleast one of fat and silicone; and an image data acquisition unitconfigured to acquire image data according to the imaging condition.

The exemplary embodiments also provide a magnetic resonance imagingapparatus comprising: an imaging condition setting unit configured toset an imaging condition applying a first suppression pulse and a secondsuppression pulse of which at least ones of types, center frequenciesand frequency bands are different from each other, the first and thesecond suppression pulses each frequency-selectively suppressing targetsignals; and an image data acquisition unit configured to acquire imagedata according to the imaging condition.

The exemplary embodiments also provide a magnetic resonance imagingapparatus comprising: an imaging unit configured to perform imagingafter applying mutually different types of fat-saturation pulse; and areconstruction unit configured to reconstruct an image based on magneticresonance signals obtained by the imaging.

The exemplary embodiments also provide a magnetic resonance imagingapparatus comprising: an imaging unit configured to perform imagingapplying an excitation pulse for exciting a magnetization of water afterapplying a first fat-saturation pulse for inclining a magnetization offat into an angle of 90 degrees to 180 degrees and a secondfat-saturation pulse for the magnetization of the fat into an angle of90 degrees to 180 degrees; and a reconstruction unit configured toreconstruct an image based on magnetic resonance signals obtained by theimaging.

The exemplary embodiments also provide a magnetic resonance imagingmethod comprising: performing imaging after applying mutually differenttypes of fat-saturation pulse; and reconstructing an image based onmagnetic resonance signals obtained by the imaging.

The magnetic resonance imaging apparatus and the magnetic resonanceimaging method as described above make it possible to performfrequency-selective-suppression for acquiring a signal that should beacquired with higher signal intensity while more satisfactorilysuppressing a signal that should be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram explaining a method for suppressing fat signalsunder a conventional CHESS method;

FIG. 2 is a diagram explaining a method for suppressing fat signalsunder a conventional SPIR method;

FIG. 3 is a diagram showing a TI for applying a pulse for excitationafter applying the SPIR pulse shown in FIG. 2;

FIG. 4 is a diagram showing an example case of insufficiently suppressedfat signals in case of performing fat-saturation under a conventionalCHESS method;

FIG. 5 is a tomographic image of an object obtained with a non-uniformstatic magnetic field under a conventional CHESS method;

FIG. 6 is a diagram showing an example case of fat signals remaining dueto an inadequately adjusted TI or FA of a fat-saturation pulse under aconventional SPIR method;

FIG. 7 is a tomographic image of an object obtained with an inadequatelyadjusted FA of a fat-saturation pulse under a conventional SPIR method;

FIG. 8 is a diagram showing a profile representing a simulation resultof effect by a conventional fat-saturation method;

FIG. 9 is a block diagram showing a magnetic resonance imaging apparatusaccording to an exemplary embodiment of the present invention;

FIG. 10 is a functional block diagram of the computer shown in FIG. 9;

FIG. 11 is a diagram showing a pulse sequence while applying an SPIRpulse and a CHESS pulse settable by the imaging condition setting unitshown in FIG. 10;

FIG. 12 is a diagram explaining a fat-saturation effect by applying theSPIR pulse and the CHESS pulse shown in FIG. 11;

FIG. 13 is a diagram showing a concrete example of pulse sequence whileapplying plural fat-saturation pulses settable by the imaging conditionsetting unit shown in FIG. 10;

FIGS. 14( a), 14(b) and 14(c) are diagrams explaining a method forsetting each FA of the first fat-saturation pulse and the secondfat-saturation pulse set by the imaging condition setting unit shown inFIG. 10;

FIGS. 15( a) and 15(b) are diagrams explaining another method forsetting each FA of the first fat-saturation pulse and the secondfat-saturation pulse set by the imaging condition setting unit shown inFIG. 10;

FIG. 16 is a diagram showing an example of sequence while applying threefat-saturation pulses set by the imaging condition setting unit shown inFIG. 10;

FIG. 17 is a diagram showing a pulse sequence while applying an SPIRpulse and two CHESS pulses having mutually different frequenciessettable by the imaging condition setting unit shown in FIG. 10;

FIG. 18 is a diagram explaining a fat-saturation effect by applying theSPIR pulse and the two CHESS pulses shown in FIG. 17;

FIG. 19 is a conceptual diagram of a pulse sequence for a frequency prepscan settable by the imaging condition setting unit shown in FIG. 10;

FIG. 20 is a diagram showing regions of data in k-space acquired whileapplying a fat-saturation pulse settable by the imaging conditionsetting unit shown in FIG. 10;

FIG. 21 is a diagram showing an example of imaging condition settingscreen displayed on the display unit shown in FIG. 10;

FIG. 22 is a flowchart showing a procedure for imaging a tomographicimage of the object with the magnetic resonance imaging apparatus shownin FIG. 9;

FIG. 23 is a tomographic image of the object obtained withfat-saturation by applying an SPIR pulse and a CHESS pulse of whichfrequencies are set to a resonance frequency of fat signals with themagnetic resonance imaging apparatus shown in FIG. 9; and

FIGS. 24( a) and 24(b) are diagrams showing a slice profile of signalintensities improved by applying plural fat-saturation pulses with themagnetic resonance imaging apparatus shown in FIG. 9.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

A magnetic resonance imaging apparatus and a magnetic resonance imagingmethod according to exemplary embodiments of the present invention willbe described with reference to the accompanying drawings.

FIG. 9 is a block diagram showing a magnetic resonance imaging apparatusaccording to an exemplary embodiment of the present invention.

A magnetic resonance imaging apparatus 20 includes a static field magnet21 for generating a static magnetic field, a shim coil 22 arrangedinside the static field magnet 21 that is cylinder-shaped, a gradientcoil 23 and an RF coil 24. The static field magnet 21, the shim coil 22,the gradient coil 23 and the RF coil 24 are built in a gantry (notshown).

The magnetic resonance imaging apparatus 20 also includes a controlsystem 25. The control system 25 includes a static magnetic field powersupply 26, a gradient power supply 27, a shim coil power supply 28, atransmitter 29, a receiver 30, a sequence controller 31 and a computer32. The gradient power supply 27 of the control system 25 includes anX-axis gradient power supply 27 x, a Y-axis gradient power supply 27 yand a Z-axis gradient power supply 27 z. The computer 32 includes aninput device 33, a display unit 34, an operation unit 35 and a storageunit 36.

The static field magnet 21 communicates with the static magnetic fieldpower supply 26. The static magnetic field power supply 26 supplieselectric current to the static field magnet 21 to get the function togenerate a static magnetic field in an imaging region. The static fieldmagnet 21 includes a superconductivity coil in many cases. The staticfield magnet 21 gets current from the static magnetic field power supply26 that communicates with the static field magnet 21 at excitation.However, once excitation has been made, the static field magnet 21 isusually isolated from the static magnetic field power supply 26. Thestatic field magnet 21 may include a permanent magnet that makes thestatic magnetic field power supply 26 unnecessary.

The static field magnet 21 has the cylinder-shaped shim coil 22coaxially inside itself. The shim coil 22 communicates with the shimcoil power supply 28. The shim coil power supply 28 supplies current tothe shim coil 22 so that the static magnetic field becomes uniform.

The gradient coil 23 includes an X-axis gradient coil 23 x, a Y-axisgradient coil 23 y and a Z-axis gradient coil 23 z. Each of the X-axisgradient coil 23 x, the Y-axis gradient coil 23 y and the Z-axisgradient coil 23 z that is cylinder-shaped is arranged inside the staticfield magnet 21. The gradient coil 23 also has a bed 37 in the areaformed inside it that is an imaging area. The bed 37 supports an objectP. Around the bed 37 of the object P, the RF coil 24 may be arrangedinstead of being built in the gantry.

The gradient coil 23 communicates with the gradient power supply 27. TheX-axis gradient coil 23 x, the Y-axis gradient coil 23 y and the Z-axisgradient coil 23 z of the gradient coil 23 communicate with the X-axisgradient power supply 27 x, the Y-axis gradient power supply 27 y andthe Z-axis gradient power supply 27 z of the gradient power supply 27,respectively.

The X-axis gradient power supply 27 x, the Y-axis gradient power supply27 y and the Z-axis gradient power supply 27 z supply currents to theX-axis gradient coil 23 x, the Y-axis gradient coil 23 y and the Z-axisgradient coil 23 z, respectively, so as to generate gradient magneticfields Gx, Gy and Gz in the X, Y and Z directions in the imaging area.

The RF coil 24 communicates with the transmitter 29 and the receiver 30.The RF coil 24 has a function to transmit a radio frequency signal givenfrom the transmitter 29 to the object P and receive an NMR signalgenerated due to a nuclear spin inside the object P that is excited bythe radio frequency signal to give to the receiver 30.

The sequence controller 31 of the control system 25 communicates withthe gradient power supply 27, the transmitter 29 and the receiver 30.The sequence controller 31 has a function to storage sequenceinformation describing control information needed in order to make thegradient power supply 27, the transmitter 29 and the receiver 30 driveand generate gradient magnetic fields Gx, Gy and Gz in the X, Y and Zdirections and a radio frequency signal by driving the gradient powersupply 27, the transmitter 29 and the receiver 30 according to apredetermined stored sequence. The above-described control informationincludes motion control information, such as intensity, impressionperiod and impression timing of the pulse electric current that shouldbe impressed to the gradient power supply 27.

The sequence controller 31 is also configured to give raw data to thecomputer 32. The raw data is complex number data obtained through thedetection of an NMR signal and A/D conversion to the NMR signal detectedin the receiver 30.

The transmitter 29 has a function to give a radio frequency signal tothe RF coil 24 in accordance with control information provided from thesequence controller 31. The receiver 30 has a function to generate rawdata that is digitized complex number data by detecting an NMR signalgiven from the RF coil 24 and performing predetermined signal processingand A/D converting to the NMR signal detected. The receiver 30 also hasa function to give the generated raw data to the sequence controller 31.

The computer 32 gets various functions by the operation unit 35executing some programs stored in the storage unit 36 of the computer32. The computer 32 may include some specific circuits instead of usingsome of the programs.

FIG. 10 is a functional block diagram of the computer 32 shown in FIG.9.

The computer 32 functions as an imaging condition setting unit 40, asequence controller control unit 41, a k-space database 42, an imagereconstruction unit 43, an image database 44, an image processing unit45 and an imaging parameter storage unit 46 by program.

The imaging condition setting unit 40 has a function to set an imagingcondition such as a pulse sequence based on instruction information fromthe input device 33 and provides the set imaging condition to thesequence controller control unit 41. Therefore, the imaging conditionsetting unit 40 has a function as an interface to display screeninformation for setting an imaging condition on the display unit 34. AGUI (Graphical User Interface) technology can be used in order toprovide the interface function mentioned above with the imagingcondition setting unit 40.

Specifically, the imaging condition setting unit 40 is configured to beable to set a pulse sequence with applications of plural different typesof frequency selective fat saturation pulses or plural same type offrequency selective fat saturation pulses under mutually differentconditions such as center frequencies and/or frequency bandwidths (rlengths), as an imaging condition. The conditions such as combination oftypes and/or the number of fat saturation pulses and center frequenciesand frequency bandwidths (π lengths) of fat saturation pulses aredetermined so that the suppression effect of fat signals improves.

FIG. 11 is a diagram showing a pulse sequence while applying an SPIRpulse and a CHESS pulse settable by the imaging condition setting unit40 shown in FIG. 10.

In FIG. 11, the abscissa denotes time. As shown in FIG. 11, a pulsesequence with application of an SPIR pulse and a CHESS pulse, forexample, as different types of fat saturation pulses can be set in theimaging condition setting unit 40. An imaging sequence that is a pulsesequence for data acquisition is set subsequently to applications of theSPIR pulse and the CHESS pulse. The pulse sequence for data acquisitionmay be a two-dimensional (2D) sequence or a three-dimensional (3D)sequence and can be set as an arbitrary type of pulse sequence such asan FE (field echo) sequence, an SE (spin echo) sequence, an FSE (fastSE) sequence, a half-Fourier FSE sequence, an EPI (echo planar imaging)sequence, and an FFE (Fast FE) sequence. Note that a half-Fourier FSEsequence using a half-Fourier method is also called FRSE (fast advancedSE or fast asymmetric SE) sequence.

The application orders of the SPIR pulse and the CHESS pulse arearbitrary. Note that the SPIR pulse is applied at the earlier timing bya TI than the start of the imaging sequence. Therefore, if the CHESSpulse is applied in the period of the TI subsequently to an applicationof the SPIR pulse, an imaging time can be shortened.

FIG. 12 is a diagram explaining a fat-saturation effect by applying theSPIR pulse and the CHESS pulse shown in FIG. 11.

In FIG. 12, the abscissa denotes a frequency and the ordinate denotes asignal intensity of an acquired NMR signal. As shown in FIG. 12, therespective resonant frequencies of fat and water have 3.5 ppmdifference. When an SPIR pulse that is a fat saturation pulse with a FAfrom 90 degrees to 180 degrees is applied while matching the frequencyof the SPIR pulse to a resonant frequency of fat, fat signals in a widerfrequency band can be suppressed than that in a case of applying a CHESSpulse. A FA and TI of the SPIR pulse are adjusted so that a longitudinalmagnetization of fat becomes the null point due to T1 relaxation atstart timing of the imaging sequence, i.e., application timing of an RFpulse for excitation of water signals, for example.

However, in the case that the adjustments of the FA and TI of the SPIRpulse are insufficient, the longitudinal magnetization of fat does notbecome the null point with only application of the SPIR pulse when theRF pulse for excitation of water signals is applied. Consequently, fatsignals remain. Therefore, fat signals remaining by applying the SPIRpulse are suppressed by an application of the CHESS pulse. In this case,a frequency of the CHESS pulse is set to the resonant frequency of fat,as well as a frequency of the SPIR pulse.

Note that when the frequency of the SPIR pulse does not match with theresonant frequency of fat signals, a center frequency of remaining fatsignals is different from the center frequency of the SPIR pulse or fatsignals remaining outside of a frequency band to be able to besuppressed by the SPIR pulse. In the above-mentioned case, therespective frequencies of the SPIR pulse and the CHESS pulse may be setto be mutually different. In addition, not only the respectivefrequencies of the SPIR pulse and the CHESS pulse, but also therespective frequency bandwidths (π lengths) thereof, can be set to bemutually different so as to more satisfactorily suppress fat signalsdepending on a spectral form of remaining fat signals.

That is, when adjustment of the TI is insufficient, to make thefrequency of the SPIR pulse match to that of the CHESS pulse isefficient on a fat saturation. On the contrary, when adjustment of theTI is satisfactory, there is a possibility to improve fat saturationeffect if the respective frequencies of the SPIR pulse and the CHESSpulse are set to be mutually different values.

Thus, when respective applications of the SPIR pulse and the CHESS pulseare combined, only fat signals over a wide range of a frequency band areexcited to be generally suppressed by the SPIR pulse with a large FA,and fat signals beyond suppression can be suppressed by the CHESS pulsewith a 90 degree FA. This allows sufficient suppression of fat signals.

As another merit in applications of plural fat saturation pulses likethe SPIR pulse and the CHESS pulse, a magnetization variation in fatdecreases according to the application number of a fat saturation pulse,and signals from fat become stable by applying a fat saturation pulseplural times. Therefore, whether or not adjustment of the TI is enough,the effect of stabilization of fat signals can be obtained by making thefrequency of the SPIR pulse match to the frequency of the CHESS pulse.In addition, as well as a combination of the SPIR pulse and the CHESSpulse, even if plural fat saturation pulses are configured with onlySPIR pulses or only CHESS pulses, the effect of stabilization of fatsignals can be obtained.

FIG. 13 is a diagram showing a concrete example of pulse sequence whileapplying plural fat-saturation pulses settable by the imaging conditionsetting unit 40 shown in FIG. 10.

In FIG. 13, the abscissa denotes time, RF&ECHO denotes RF pulses andecho signals, Gse denotes gradient magnetic field pulses for SE (sliceencode), Gro denotes gradient magnetic field pulses for RO (readout),and Gpe denotes gradient magnetic field pulses for PE (phase encode).

As shown in FIG. 13, the second fat saturation pulse (2nd fat sat pulse)with a β-degree FA to tilt the magnetization of fat is applied at thetiming to pass the time TI1 from the first fat saturation pulse (1st fatsat pulse) with an α-degree FA to tilt the magnetization of fat. Thegradient magnetic field spoiler pulses to defuse transversemagnetizations respectively are applied subsequently to applying thefirst and the second fat saturation pulses. As described below, thegradient magnetic field spoiler pulse may be applied only after applyingthe second fat saturation pulse. An imaging sequence is set subsequentlyto applications of the first and the second fat saturation pulses.

FIG. 13 shows an example of setting a three-dimensional multi-slice FFEsequence to perform data acquisition under the segment k-space method asthe imaging sequence. Therefore, a sequence to acquire data in eachsegment is repeated at an interval of an adjustment time (recoverytime). That is, there is a space time that is the recovery time amongdata acquisition sequences corresponding to respective segments. Thedata acquisition in a common segment is performed by a sequence torepeat application of an imaging pulse n times with a repetition time(TR). Then, echo signals are acquired subsequently to applying imagingpulses. An imaging pulse is an excitation pulse for excitingmagnetization of water and, therefore, the acquired echo signal is anecho signal from water.

Although the first fat saturation pulse and the second fat saturationpulse can be applied respectively precedent to data acquisition of eachsegment, as shown in FIG. 13, applying the first fat saturation pulseand the second fat saturation pulse only precedent to data acquisitionof a specific segment leads to reduction of an imaging period.Preferably, the pulse sequence is set so that the second fat saturationpulse is applied at the timing before the period TI2 from an applicationtime of an imaging pulse to acquire data near the center of k-space. Bydoing this, increase of an imaging period can be suppressed whileperforming a sufficient fat saturation to data, near the center ofk-space, to which more satisfactory fat saturation effect is required.

Therefore, when the imaging sequence is a three-dimensional FFEsequence, it is preferable to use a sequence to perform data acquisitionfrom the vicinity of the center of k-space at the beginning, such asinterleave acquisition or centric acquisition.

The interleave acquisition is a data acquisition method to acquire datafrom the vicinity of the center in k-space toward the edge in a PEdirection and to acquire data from the edge linearly in a SE direction.The centric acquisition is a data acquisition method to acquire datafrom the vicinity of the center in k-space toward the edge in the bothdirections of PE and SE. Note that, as another data acquisition method,the sequential acquisition to acquire data from the edge of k-spacelinearly in both directions of PE and SE is known.

Here, a determination method of a FA will be described.

FIGS. 14( a), 14(b) and 14(c) are diagrams explaining a method forsetting each FA of the first fat-saturation pulse and the secondfat-saturation pulse set by the imaging condition setting unit 40 shownin FIG. 10.

FIG. 14( a) shows the first fat-saturation pulse, the secondfat-saturation pulse and an imaging pulse, for acquiring data on thecenter of k-space, applied subsequently to the second fat-saturationpulse. FIG. 14( b) shows a variation of T1 in case of each part having amutually different T1 value in fat and T1 relaxing. FIG. 14( c) shows avariation of T1 in case where apparent a T1 shows plural differentvalues due to non-uniform RF pulses although a T1 value of fat isconstant.

As shown in FIG. 14( a), after the first fat saturation pulse isapplied, in the case that T1 values are mutually different even in a fatregion due to the influence of surrounding tissues, mutually differentT1 relaxations occur as shown in FIG. 14( b). As shown in FIG. 14( c),in the case that apparent T1 values are mutually different due to anuneven RF pulse even if T1 values are same, mutually different T1relaxations occur.

Therefore, when the FA of the first fat saturation pulse is set at 90degrees or more (α≧90°) so that the T1 becomes the null point byapplying the second fat saturation pulse at the timing to pass theperiod TI1 subsequently to application of the first fat saturationpulse, mutually different T1 relaxations can be cancelled once. That is,as shown in FIG. 14( b), even if TI values of fat are mutually differentin each region, the difference between longitudinal magnetizations isreduced by an application of the second fat saturation pulse andnon-uniformity of fat saturation is improved. As shown in FIG. 14( c),even if an application of an uneven RF pulse results in a differencebetween longitudinal magnetizations, T1 recoveries without a differencecan be obtained by applying the second fat saturation pulse.

The FA of the first fat saturation pulse is set to a large angle fromapproximately 100 degrees to 130 degrees (α=100-130 degrees), forexample, since the T1 becomes the null point by the application of thesecond fat saturation pulse. That is to say, the FA of the first fatsaturation pulse can be determined according to the period TI1 betweenthe application times of the first fat saturation pulse and the secondfat saturation pulse so that the T1 becomes the null point at the centerof a waveform of the second fat saturation pulse. In addition, the FA ofthe first fat saturation pulse can be adjusted in the range from 90degrees to 180 degrees according to the degree of suppression of fatsignals. Then, the first fat saturation pulse to tilt a magnetization offat into an angle from 90 degrees to 180 degrees can be set.

When the period TI2 from the application time of the second fatsaturation pulse until the application time of the imaging pulse toacquire data near the center in k-space is sufficiently short, even ifthe FA of the second fat saturation pulse is set to 90 degrees (β=90°),sufficient fat saturation effect can be obtained at the applicationtiming of the imaging pulse. Therefore, the second fat saturation pulseto tilt a magnetization of fat by 90 degrees can be set. In an FFEsequence to obtain a T1 weighted image by the interleave acquisition orthe centric acquisition, the period T12 from an application time of thesecond fat saturation pulse until an application time of the imagingpulse to acquire data near the center in k-space is sufficiently short.Then, the shorter the period TI2 mentioned above is, the more fatsaturation effect improves by reducing the recovery of fat signals.

On the other hand, when the period TI2 from an application time of thesecond fat saturation pulse until an application time of the imagingpulse to acquire data near the center in k-space is not sufficientlyshort, satisfactory fat saturation effect can be obtained at applicationtiming of the imaging pulse if β-degree FA of the second fat saturationpulse is set so that the T1 becomes the null point at the applicationtime of the imaging pulse.

Though the imaging sequence shown in FIG. 13 is an FFE sequence, whenthe imaging sequence is a sequence to obtain a T2 (transverserelaxation) weighted image such as an FE sequence, an SE sequence, anFSE sequence, and an FASE sequence, it is preferable to set the FA β ofthe second fat saturation pulse so that the T1 becomes the null point atthe application time of the imaging pulse since the period TI2 from theapplication time of the second fat saturation pulse to the applicationtime of the imaging pulse to acquire data near the center in k-space islong generally. In the case of performing the sequential acquisition byan FFE sequence, since the period TI2 from the application time of thesecond fat saturation pulse to an application time of the imaging pulseto acquire data near the center in k-space is long, it is alsopreferable to set the FA p of the second fat saturation pulse so thatthe T1 becomes the null point at the application time of the imagingpulse.

That is to say, it is preferable that the FA β of the second fatsaturation pulse be set to an appropriate value according to a type ofan imaging sequence.

FIGS. 15( a) and 15(b) are diagrams explaining another method forsetting each FA of the first fat-saturation pulse and the secondfat-saturation pulse set by the imaging condition setting unit 40 shownin FIG. 10.

FIG. 15( a) shows the first fat-saturation pulse, the secondfat-saturation pulse and an imaging pulse, for acquiring data on thecenter of k-space, applied subsequently to the second fat-saturationpulse. FIG. 15( b) shows a variation of T1 in case of fat T1-relaxingwith a constant T1 value.

When the imaging sequence is a sequence to obtain a T2 weighted imagesuch as an FSE sequence or an FFE sequence to perform the sequentialacquisition, the period TI2 from an application time of the second fatsaturation pulse to an application time of the imaging pulse to acquiredata near the center in k-space becomes long. In this case, when the FAβ of the second fat saturation pulse is set to 90 degrees, there is apossibility that fat signals recover subsequently to the application ofthe second fat saturation pulse and a recovery amount of fat signalscannot be ignored at the point of an application of the imaging pulse.Therefore, as shown in FIGS. 15( a) and 15(b), it is preferable that theFA β of the second fat saturation pulse be set to over 90 degrees (β>90°and the T1 value becomes the null point at application timing of theimaging pulse.

In this case, the FA β of the second fat saturation pulse is determinedaccording to respective values of the FA α of the first fat saturationpulse, the period TI1 between application times of the first fatsaturation pulse and the second fat saturation pulse, and the period TI2between application times of the second fat saturation pulse and theimaging pulse. The FA β of the second fat saturation pulse is alsodetermined depending on the T1 value of fat. Therefore, the FA β of thesecond fat saturation pulse is obtained as β=F (α, TI1, TI2, T1 value)using a function F. That is to say, parameters α, β, TI1, TI2 can beadjusted so that the T1 value becomes the null point at an applicationtime of the imaging pulse.

A specific example of each FA is to apply an SPIR pulse with a 150degree FA (α=150°) as the first fat saturation pulse and to apply aCHESS pulse with a 95 degree FA (β=95°) as the second fat saturationpulse when an FSE sequence is used for data acquisition.

Incidentally, three fat saturation pulses or above can be also applied.While increase of an imaging period can be suppressed to the minimumwhen the number of fat saturation pulses is set to two, when the numberof fat saturation pulses is set to three or more, fat saturation effectcan be improved by stability of fat signals. Then, the determinationmethods of each FA as described above can be used similarly in the caseof using three fat saturation pulses or more.

FIG. 16 is a diagram showing an example of sequence while applying threefat-saturation pulses set by the imaging condition setting unit 40 shownin FIG. 10.

In FIG. 16, the abscissa denotes time, RF denotes RF pulses, Gse denotesgradient magnetic field pulses for slice encode, Gro denotes gradientmagnetic field pulses for readout, and Gpe denotes gradient magneticfield pulses for phase encode.

As shown in FIG. 16, it is also possible to apply the second fatsaturation pulse after passing the period TI1 from an application of thefirst fat saturation pulse and, furthermore, to apply the third fatsaturation pulse after passing the period TI2 from an application of thesecond fat saturation pulse. Then, an imaging pulse to acquire data nearthe center in k-space is applied after passing the period TI3 from theapplication of the third fat saturation pulse. Gradient magnetic fieldspoiler pulses to suppress unnecessary transverse magnetization elementsare applied subsequently to application of each fat saturation pulse.

Note that the gradient magnetic field spoiler pulses applied after thefirst and the second fat saturation pulses may be omitted. That is, animaging period can be reduced while maintaining the suppression effectof a transverse magnetization to some extent by applying a gradientmagnetic field spoiler pulse only subsequently to the application of thelast fat saturation pulse.

As shown in FIG. 16, when multiple fat saturation pulses are applied,the merit of stability of fat signals can be obtained since a fatsaturation state is repeated, the variation of a magnetization of fatbecomes small gradually, and a steady state can be obtained.

When three or more fat saturation pulses are applied, the FA of everyfat saturation pulse or each fat saturation pulse after the second canbe set to 90 degrees. Note that fat saturation effect can be improved byadjusting appropriately a FA so as to be over 90 degrees, according tofat saturation effect.

It is preferable that every frequency of fat saturation pulses is set tothe resonant frequency of fat to improve stability of a magnetization offat. Note that, when a magnetic field is uneven locally, another peakoften appears at a position shifted from the original resonant frequencyof fat on a frequency spectrum. In this case, satisfactory fatsaturation effect can be obtained by offsetting frequencies of some fatsaturation pulses so as to match the frequency corresponding to a peak,even to an uneven magnetic field locally.

FIG. 17 is a diagram showing a pulse sequence while applying an SPIRpulse and two CHESS pulses having mutually different frequenciessettable by the imaging condition setting unit 40 shown in FIG. 10.

In FIG. 17, the abscissa denotes time. As shown in FIG. 17, a pulsesequence by which an SPIR pulse and a CHESS pulse as different types offat saturation pulses and two CHESS pulses having mutually differentfrequencies as same type of fat saturation pulses having differentconditions mutually are applied respectively can be set in the imagingcondition setting unit 40. An imaging sequence that is a pulse sequencefor data acquisition is set subsequently to applying the SPIR pulse andthe two CHESS pulses.

Both of the frequency f1 of the SPIR pulse and the frequency f1 of theCHESS pulse 1, which is one of the two CHESS pulses, are set to aresonant frequency of fat signals and the frequency f2 of the CHESSpulse 2, which is the other CHESS pulse, is set to a different valuefrom the frequency f1 of the SPIR pulse. Note that the respectivefrequencies of the SPIR pulse and the two CHESS pulses may be set tomutually different frequencies according to a frequency spectral shapeof fat signals.

The application order of the SPIR pulse and the two CHESS pulses isarbitrary. Note that the SPIR pulse is applied at the earlier timing bya TI than the beginning of the imaging sequence. Therefore, to when thetwo CHESS pulses are applied during the period TI subsequently toapplying the SPIR pulse, the imaging period can be reduced.

FIG. 18 is a diagram explaining a fat-saturation effect by applying theSPIR pulse and the two CHESS pulses shown in FIG. 17.

In FIG. 18, the abscissa denotes a frequency and the ordinate denotes asignal intensity of an acquired NMR signal. For example, there is a casethat a static magnetic field is uneven locally like a case to beaffected strongly by susceptibility or a case to be affected by shapesuch as a jaw region and a breast region under the high magnetic fieldsuch as 3T. When a static magnetic field is uneven, as shown in FIG. 18,there is a case that another peak appears at a different frequency fromthe original resonant frequency by shifting of a resonant frequency offat signals. In this case, even if the fat signals spreading in the bandnear the original resonant frequency of fat signals are suppressed byapplying the SPIR pulse and the CHESS pulse having a frequency set tothe resonant frequency f1 of fat signals as shown in FIG. 12, there is acase that fat signals still remain in another frequency band.

Therefore, the CHESS pulse 2 having the frequency f2 shifted from theresonant frequency of fat signals is applied subsequently to applyingthe SPIR pulse and the CHESS pulse 1 having the frequency f1 set to theresonant frequency of fat signals. The frequency f2 of the CHESS pulse 2is set to the value offset from the original resonant frequency of thefat signals so that remaining fat signals after applying the SPIR pulseand the CHESS pulse 1 are sufficiently suppressed. Consequently, eventhe case that the resonant frequency of fat signals shifts, fatsaturation effect can be obtained sufficiently.

Another example of application of plural fat saturation pulses is a caseof applying two CHESS pulses. That is, the SPIR pulse may be omitted inthe pulse sequence shown in FIG. 17. In this case, when fat signals showplural peaks, fat saturation effect can be improved by setting therespective frequencies of the two CHESS pulses to different valuesmutually. When the center frequencies of the two CHESS pulses are set tobe same, there is the case that fat saturation effect is improved bysetting the frequency bandwidths of the two CHESS pulses to be differentfrom each other.

As described above, the imaging condition setting unit 40 can setimaging conditions for an imaging scan to acquire a diagnostic image. Inaddition, a prescan can be performed separately to obtain a frequency ofa CHESS pulse necessary to determine imaging conditions for an imagingscan. A prescan is performed precedent to an imaging scan and performsdata acquisition plural times while changing a frequency of a CHESSpulse. The prescan to acquire data while changing a frequency asdescribed above is called a frequency prep scan here.

When a frequency prep scan is performed, the imaging condition settingunit 40 can set an imaging condition for the frequency prep scan.

FIG. 19 is a conceptual diagram of a pulse sequence for a frequency prepscan settable by the imaging condition setting unit 40 shown in FIG. 10.

In FIG. 19, the abscissa denotes time. As shown in FIG. 17, when theimaging scan is performed using the pulse sequence with applications ofan SPIR pulse and two CHESS pulses having mutually differentfrequencies, it is necessary to obtain the frequency f2 of the CHESSpulse while offsetting a frequency from the resonant frequency of fat. Apulse sequence for the frequency prep scan to acquire data over N timeswith changing the frequency f2 of the CHESS pulse while offsetting afrequency from the resonant frequency of fat to f21, f22, . . . , andf2N can be set.

The number N and the variation width of the frequency f2 of the CHESSpulse to be changed can be set arbitrarily by providing instructioninformation from the input device 33 to the imaging condition settingunit 40.

It is preferable that an imaging sequence in a pulse sequence for afrequency prep scan is equivalent to an imaging sequence in an imagingscan in view of determining an appropriate frequency f2 of the CHESSpulse. Note that it is preferable that a sequence for a frequency prepscan be set to a two-dimensional sequence for reduction of an imagingperiod.

In a pulse sequence for a frequency prep scan, the applications of anSPIR pulse and/or another CHESS pulse of which frequency is not adetermination target may be omitted.

Plural images corresponding to different frequencies of a CHESS pulseare obtained by performing the pulse sequence for the frequency prepscan set above and an appropriate frequency of a CHESS pulse can bedetermined by selecting the image having most preferable contrast for auser through visual observation, for example.

On the other hand, a frequency of a fat saturation pulse can also bedetermined based on a frequency spectrum obtained by performing aprescan for acquiring the frequency spectrum without a frequency prepscan. That is, if a frequency spectrum is acquired precedent to animaging scan for a fat saturation image, a frequency offset of fatsignals can be recognized. The offset of a fat saturation pulse can bedetermined so as to match with the frequency offset of fat signals onthe frequency spectrum.

Thus, when the frequency of a fat saturation pulse is determinedsynchronizing with the frequency offset of fat signals obtained based ona frequency spectrum, the influence of an uneven magnetic field locallycan be reduced and fat saturation effect can be obtained with stability.

Offsetting a frequency of fat signals can be performed manually bydisplaying a frequency spectrum on the display unit 34 and designatingthe frequency corresponding to the shifted peak of fat signals with theoperation of the input device 33. Further, a peak position of fatsignals and a shifted frequency of fat signals can be also detectedautomatically. In the case that a peak position of fat signals isdetected automatically, for example, a range in the frequency directionin which the peak of fat signals is estimated to exist is designatedbased on the peak of water signals and a signal value over a thresholdin the designated range is detected. A shift amount of the resonantfrequency of fat signals can be detected automatically by measuring afrequency difference between the peak corresponding to the originalresonant frequency of fat signals and the other peak.

Note that performing a scan for acquiring a frequency spectrum under astate in which fat signals are suppressed by applying a non-selectiveinversion recovery (IR) pulse makes it possible to obtain a frequencyspectrum having a maximum value equivalent to the peak of water signals.A range for detecting the peak of fat signals in the frequency directioncan be set based on the peak of water signals.

The parameters of a fat saturation pulse as described above including aFA, α, β, pulse intervals TI1, TI2, TI3, a frequency, and a frequencybandwidth (τ length) can be set manually or automatically by operatingthe input device 33 through the screen for setting imaging conditionsdisplayed on the display unit 34.

Respective parameters can be obtained for each imaging condition such asa type of sequence, a data acquisition method and/or an imaging regionin advance by a test imaging so as to be able to set a parameter orparameters of a fat saturation pulse automatically. In this case, therespective parameters are obtained while relating to one or more imagingconditions such as a type of sequence, a data acquisition method and/oran imaging region. Then, the obtained parameters with regard to a fatsaturation pulse are stored in the imaging parameter storage unit 46.

Then, by storing a preferred combination of parameters and/or theoptimum parameters related with every imaging region in the imagingparameter storage unit 46, one or more parameters of each fat saturationpulse such as a FA, a pulse interval, a frequency and/or a frequencybandwidth (τ length) are read from the imaging parameter storage unit 46to the imaging condition setting unit 40 to be set as imaging conditionsautomatically only by inputting information for designating an imagingregion and selecting a combination of parameters according to a type ofsequence and a data acquisition method while operating the input device33.

For example, there is the case that a frequency of the fat signals notto be suppressed sufficiently depending on a FA of a certain fatsaturation pulse, that is, a frequency of a fat saturation pulse to beadded is determined. Accordingly, an appropriate frequency of anotherfat saturation pulse according to each FA of the certain fat saturationpulse can be obtained to each imaging region in advance by a testimaging. Then, it possible to configure so that the imaging conditionsetting unit 40 determines a frequency of another fat saturation pulseaccording to a FA of a certain fat saturation pulse as an imagingcondition automatically when an imaging region is selected with theoperation of the input device 33.

Similarly, it is possible to determine a FA of another fat saturationpulse according to a FA of a certain fat saturation pulse. As anotherexample, a FA and/or a frequency of another fat saturation pulse can bedetermined according to the period from an application of a certain fatsaturation pulse to an application of an excitation pulse.

In addition to this, conditions with regard to a fat saturation pulseinclude a data region in k-space to be acquired with application of afat saturation pulse.

FIG. 20 is a diagram showing regions of data in k-space acquired whileapplying a fat-saturation pulse settable by the imaging conditionsetting unit 40 shown in FIG. 10.

In FIG. 20, the abscissa denotes RO direction of k-space and theordinate denotes PE direction of the k-space.

As shown in FIG. 20, data in k-space is divided into data in alow-frequency region near the center (k0) of k-space in the PE directionand data in a high-frequency region at edge part away from the center ofk-space in the PE direction. Then, it is in data near the center ofk-space that more satisfactory fat saturation effect is desired. Bysetting imaging conditions so as to acquire only data near the center ofk-space with applications of plural fat saturation pulses, on the otherhand, so as to acquire data at the part away from the center of k-spacewithout an application of a fat saturation pulse or with an applicationof one fat saturation pulse, an imaging period can be reduced.

Specifically, when a sequential acquisition is performed with an SEsequence or an FE sequence, there is the case that increase of animaging period is remarkable by applying plural fat saturation pulsesevery encode. Then, for example, setting the imaging conditions so as toacquire data near the center of k-space with applications of both anSPIR pulse and a CHESS pulse, on the other hand, so as to acquire datain the region away from the center of k-space with application of only asingle SPIR pulse or CHESS pulse makes it possible to suppress increaseof an imaging period.

Then, respective imaging conditions as described above can be set withoperation of the input device 33 through the screen for setting imagingconditions displayed on the display unit 34.

FIG. 21 is a diagram showing an example of imaging condition settingscreen displayed on the display unit 34 shown in FIG. 10.

As shown in FIG. 21, the screen for setting imaging conditions isconfigured by a GUI technology. FIG. 21 shows an example of screen forsetting imaging conditions in the case that a center frequency of a fatsaturation pulse is set based on the frequency spectrum obtained by ascan for acquiring a frequency spectrum. Therefore, a frequency spectrumobtained by a prescan is displayed on the screen for setting imagingconditions.

When a scan for acquiring a frequency spectrum is performed whilesuppressing fat signals by applying a non-selective IR pulse, afrequency spectrum having a maximum value equivalent to the peak ofwater signals as shown in FIG. 21 can be obtained. Then, when a constantfrequency band (fb) is set from the peak of water signals and a peak ofsignals is detected automatically in the set frequency band, a peak offat signals or a locally shifted peak of fat signals can be detectedautomatically. A frequency of fat signals detected automatically can bealso set automatically as a frequency of a desired fat saturation pulse.Further, a user can also select a frequency corresponding to a peak offat signals as a frequency of a fat saturation pulse with the inputdevice 33 such as a mouse.

In addition, the number and/or types of fat saturation pulses can bealso set arbitrarily. In an example shown in FIG. 21, two fat saturationpulses of an SPIR pulse and a CHESS pulse are set. In addition, a centerfrequency and a FA of each fat saturation pulse can be adjustedrespectively. For example, each of frequency offset and FA of a fatsaturation pulse can be set to a desired value by operating a slide bar(scroll bar).

Furthermore, whether or not a fat saturation pulse is applied in ahigh-frequency region in k-space can be set through a screen for settingimaging conditions. In the example in FIG. 21, an imaging condition inwhich a fat saturation pulse is not applied in a high-frequency regionis selected. In addition to this, as in the case of determining afrequency offset of a fat saturation pulse by performing a frequencyprep scan, various types of interface can be generated according to amethod of setting imaging conditions.

Then, other functions of the computer 32 will be described.

The sequence controller control unit 41 of the computer 32 has afunction for controlling the driving of the sequence controller 31 bygiving a pulse sequence, acquired from the imaging condition settingunit 40, to the sequence controller 31 based on information from theinput device 33 or another element and a function for receiving raw datathat is k-space (Fourier space) data from the sequence controller 31 andarranging the raw data to k-space formed in the k-space database 42.

The image reconstruction unit 43 has a function for generating imagedata from k-space data by capturing the k-space data from the k-spacedatabase 42 and performing image reconstruction processing such as two-or three-dimensional Fourier transform processing to the k-space data,and writing the generated image data to the image database 44.

The image processing unit has a function for performing necessary imageprocessing to image data read from the image database 44 and displayingthe image data on the display unit 34.

Next, the operation and action of a magnetic resonance imaging apparatus20 will be described.

FIG. 22 is a flowchart showing a procedure for imaging a tomographicimage of the object P with the magnetic resonance imaging apparatus 20shown in FIG. 9. The symbols including S with a number in FIG. 22indicate each step of the flowchart.

Here, an example of determining a frequency offset of a fat saturationpulse while performing a frequency prep scan will be described.

First, the object P is set to the bed 37, and a static magnetic field isgenerated at an imaging area of the magnet 21 (a superconducting magnet)for static magnetic field excited by the static-magnetic-field powersupply 26. Further, the shim-coil power supply 28 supplies current tothe shim coil 22, thereby uniformizing the static magnetic fieldgenerated at the imaging area.

Next, in step S1, a frequency prep scan is performed as needed. Afrequency prep scan is efficient when a CHESS pulse is needed to beapplied with a frequency shifted from a resonant frequency of fatsignals and an appropriate frequency of the CHESS pulse is unknown, asin a case where non-uniformity of a static magnetic field is notimproved even by shimming.

Therefore, the imaging conditions including a pulse sequence for afrequency prep scan as shown in FIG. 19 are set. That is, the variationnumber N and/or the variation width of a frequency f2 of a CHESS pulseis set according to instruction information from the input device 33 inthe imaging condition setting unit 40. Then, when a user providesinstruction information for performing the frequency prep scan from theinput device 33 to the sequence controller control unit 41, the imagingconditions including the pulse sequence for the frequency prep scan areprovided to the sequence controller 31 from the imaging conditionsetting unit 40 through the sequence controller control unit 41.

Then, the sequence controller 31 drives the gradient power supply 27,the transmitter 29, and the receiver 30 in accordance with the sequencefor the frequency prep scan, thereby generating a X-axis gradientmagnetic field Gx, a Y-axis gradient magnetic field Gy, a Z-axisgradient magnetic field Gz and RF signals. Then, the RF coil 24 receivesNMR signals generated due to nuclear magnetic resonance of proton spinsin the object P. The receiver 30 receives the NMR signals from the RFcoil 24 and generates raw data that is digital data of the NMR signal.The receiver 30 supplies the generated raw data to the sequencecontroller 31. The raw data is arranged as k-space data in the k-spacegenerated in the k-space database 42 by the sequence controller controlunit 41.

The image reconstruction unit 43 reads the k-space data from the k-spacedatabase 42 and performs image reconstruction processing to the readk-space data, thereby generating image data. The generated image data iswritten in the image database 44 by the image reconstruction unit 43.Further, the image processing unit 45 performs necessary imageprocessing to the image data read from the image database 44 anddisplays the data on the display unit 34.

Consequently, plural fat-saturated images by applying CHESS pulseshaving mutually different frequencies are displayed on the display unit.

Next, in step S2, when a user selects the image fat-saturated mostsatisfactory out of the displayed plural images by operation of theinput device 33, selection information of the image is provided to theimaging condition setting unit 40 from the input device 33. Then, thefrequency of the CHESS pulse corresponding to the selected image is setas a frequency of a CHESS pulse used in an imaging scan in the imagingcondition setting unit 40. That is, a frequency of a CHESS pulse isdetermined.

Then, in step S3, a combination of fat saturation pulses used in theimaging scan is determined. That is, instruction information indicatingthe conditions including a type, a frequency and a frequency bandwidthof a fat saturation pulse is provided from the input device 33 to theimaging condition setting unit 40.

Then, in step S4, instruction information of an imaging sequence used inthe imaging scan is provided from the input device 33 to the imagingcondition setting unit 40. Therefore, for example, a pulse sequence withapplications of an SPIR pulse and two CHESS pulses that have differentfrequencies mutually as shown in FIG. 17 is set as an imaging conditionfor the imaging scan. The frequency f1 of the SPIR pulse and one CHESSpulse is set to the resonant frequency of fat signals and the frequencyf2 of the other CHESS pulse is set to the frequency determined byperforming the frequency prep scan.

Then, in step S5, the imaging scan is performed. That is, the inputdevice 33 provides an instruction for performing the imaging scan to thesequence controller control unit 41 and the imaging scan is performedunder a flow similar to that in case of the frequency prep scan.Consequently, k-space data acquired by the imaging scan is stored in thek-space database 42.

Then, in step S6, the image reconstruction unit 43 generates image datafrom the k-space data acquired by the imaging scan and an image obtaineddue to the imaging scan through image processing in the image processingunit 45 is displayed on the display unit 34.

Here, the image shown in the display unit 34 is obtained by suppressingfat signals in a wider frequency band corresponding to the SPIR pulseand by the CHESS pulse suppressing fat signals beyond suppression by theSPIR pulse. Furthermore, even if a static magnetic field is uneven and aresonant frequency of fat signals shifts, fat signals are suppressed byapplication of the CHESS pulse of which frequency is offsetappropriately. Consequently, the image in which fat is suppressedsatisfactorily is displayed on the display unit 34.

FIG. 23 is a tomographic image of the object P obtained withfat-saturation by applying an SPIR pulse and a CHESS pulse of whichfrequencies are set to a resonance frequency of fat signals with themagnetic resonance imaging apparatus 20 shown in FIG. 9.

As shown in FIG. 23, it is confirmed to suppress fat satisfactorily byapplications of the SPIR pulse and the CHESS pulse.

That is, the magnetic resonant imaging apparatus 20 described abovemakes fat saturation effect improved by combining plural fatsaturations. For example, fat signals in a wider frequency band can besuppressed by an SPIR pulse and remaining fat signals can be suppressedby a CHESS pulse additionally. For example, applying plural CHESS pulseshaving mutually different frequencies can make fat saturation effectimproved in a region, such as a jaw, in which a resonant frequency offat signals is shifted due to an uneven static magnetic field.

Therefore, a fat saturation method with combining plural frequencyselective fat saturation pulses is a fat saturation method alternativeto the STIR method that is a conventional non-frequency selective fatsaturation method. In addition, in the STIR method, a dead time TIduring which a longitudinal magnetization of fat protons becomes thenull point is needed after applying an excitation pulse. Additionally,signal intensities of water signals become low in order to excite allsignals including both fat signals and water signals.

Therefore, a fat saturation method with combining plural frequencyselective fat saturation pulses makes it possible to reduce an imagingperiod and increase intensities of water signals compared to the STIRmethod.

Further, since the effect to constantly maintain a state in which fatsignals are suppressed is obtained by applying fat saturation pulsescontinuously, a slice profile representing a signal intensity of anexcitation pulse to a frequency variation can be improved.

FIGS. 24( a), 24(b) and 24(c) are diagrams showing a slice profile ofsignal intensities improved by applying plural fat-saturation pulseswith the magnetic resonance imaging apparatus 20 shown in FIG. 9.

FIG. 24( a) shows a slice profile of signal intensities obtained by aconventional fat-saturation method while applying one CHESS pulse. FIG.24( b) shows a slice profile of signal intensities obtained by afat-saturation method while applying both an SPIR pulse and a CHESSpulse. Therefore, in FIGS. 24( a) and 24(b), each abscissa denotes afrequency and each ordinate denotes a relative signal intensity.

It can be recognized that a slice profile is improved in a case ofapplying both an SPIR pulse and a CHESS pulse shown in FIG. 24( b)compared to a slice profile in a case of applying one CHESS pulse shownin FIG. 24( a).

That is, by applying continuously two fat saturation pulses whether theyare the same type or different types, a profile is improved compared tothat in a case of applying a single fat saturation pulse. When dataacquisition is performed under the segmented k-space method, it ispossible to maintain a constant profile without depending on theapplication interval of a fat saturation pulse varying depending on thenumber of segments. Consequently, even if the application interval of afat saturation pulse is short, sufficient saturation effect isfavorable. Since suppressed signals do not invert even if a FA of a fatsaturation pulse is changed, an uneven suppression is improved.

That is, since a longitudinal magnetization of signals to be desired tobe suppressed can reduce sufficiently by applying plural fat saturationpulses continuously in high-speed imaging, the influence of thelongitudinal magnetization can be suppressed. Consequently, a profilecan be stabilized and it is possible to make fat saturation effect inthe frequency direction more uniform. In addition, since influence dueto non-uniformity of a magnetic field and an RF pulse can be reduced, afrequency band of suppressed signals can be extended in some degreecompared to that in a case of applying one fat saturation pulse.

Note that, in the embodiment described above, although fat signals aresuppression targets, plural fat saturation pulses by which targetsignals from a matter except fat are to be suppressed can be alsoapplied. For example, signals from silicone may be suppression targetsand, alternatively, signals not from fat, but water, may be alsosuppression targets. For example, in a case of performing watersaturation, a frequency of a saturation pulse is set to the resonantfrequency of water.

A water saturation pulse can be used as a non-selective IR pulse whenunnecessary signals from a blood vessel are to be suppressed or whenlabeling is performed with a Time-SLIP (time spatial labeling inversionpulse) method. The Time-SLIP method is a technology for depicting bloodsignals as high signals or as low signals by labeling a certain spacearea with a constant interval.

More specifically, in the Time-SLIP method, a Time-SLIP pulse is appliedafter a constant delay from an R wave of an ECG (electrocardiogram)signal, as needed. The Time-SLIP pulse consists of an area non-selectiveIR pulse and an area selective IR pulse. When blood flowing into animaging area is labeled by an area selective IR pulse, a signalintensity of a part where the blood reaches after a period becomes highand, consequently, blood information can be imaged. When an ECGsynchronized imaging is performed, an ECG unit 38 to acquire an ECGsignal from the object P is set as shown in FIG. 9. Then, an ECG signalacquired from the ECG unit 38 is used for an ECG synchronized imaging.

What is claimed is:
 1. A magnetic resonance imaging (MRI) apparatuscomprising: an MRI imaging condition setting unit configured to set animaging condition frequency-selectively applying a first suppressionpulse for suppressing fat and further frequency-selectively applying asecond suppression pulse to the fat after applying the first suppressionpulse, a slip angle of the second suppression pulse differing from thatof the first suppression angle, and the second suppression pulse furthersuppressing remaining fat after applying the first suppression pulse;and an image data acquisition unit configured to acquire image dataaccording to the imaging condition.